In a conventional injection molding process, the thermoplastic resin is injected at high temperature and high pressure in a closed mold, allowing complex shapes to be obtained. With these high pressure and temperature injection techniques, very compact and very rigid parts can be obtained, so many times reinforcing elements are not needed. These processes allow introducing inserts or decorative linings inside the mold.
Nevertheless, these injection processes have certain drawbacks, for example:                It is necessary to use high quality materials to reach the required mechanical and/or structural characteristics, as well as the desired external appearance. Many times it is not possible to use recycled or low quality materials.        It is necessary to closely control the temperature so that the material flows well. When plastic cools it contracts, so it is necessary to closely control the temperature, especially when trying to obtain complex shapes.        The high temperature involves enormous energy consumption. The required facility furthermore is more complex since it must be able to heat the material at that high temperature. In addition, when the use of inserts or reinforcements introduced in the mold is required or when the part to be obtained must incorporate a lining, it is necessary to take into account that not all the materials used to form inserts, reinforcements or linings can withstand these high temperatures.        The injection nozzle is in a specific area of the mold, therefore in order to reach the most distant areas and/or areas with complex shapes, the material has to flow very well. In order to obtain completely filling the mold and a good packing of the material (which allows improving the mechanical and structural behavior of the obtained part), it is necessary to use high pressures. These necessary high pressures make a more complex facility necessary.        
In some cases (as in the case of roof panels or door panels for automotive vehicles), the plastic part must incorporate a lining layer on at least one of the two faces of the obtained part (generally arranged on the “visible face”, i.e. on the face of the product that is visible to the user during conventional product use). In order for this lining layer to adhere to the plastic part, the lining layer is generally placed in the mold and then the thermoplastic material is injected. This has the drawback that it is not possible to use any lining material since many materials may degrade (for example due to burning or glares which deteriorate their appearance or involve the loss of their initial properties) due to the high temperatures of the injection. It is therefore occasionally necessary to use a complementary protection layer that is placed between the lining layer and the surface of the thermoplastic material and functions as insulation against the high temperature of the injection process, to prevent deterioration of the lining layer. However, this increases the complexity and increases the final cost of the process. Furthermore, the use of the complementary protection layer is sometimes not enough to completely solve these problems, which may require the use of special (and/or expensive) materials for the lining layer.
The high pressure of the injection may further break the lining material.
The movement of the plastic material flowing from the injection nozzle throughout the mold may furthermore cause the lining layer to move, wrinkle or become detached, dragged by the movement of the plastic material, possibly changing the final appearance of the part.
The injection processes can be carried out in a closed mold or in an open mold (in which case the injection is carried out in two steps: first part of the material is injected with the mold open, then the mold is closed and the injection is then finished).
Another technique that can be used for producing thermoplastic parts is called open mold compression. This technique is traditionally based on introducing the plastic material in an open mold, closing with the corresponding upper mold and applying a closing pressure. In this technique, the pressures are not as high as in injection molding processes, and the plastic material is usually at a lower temperature than the temperature used in injection processes.
These open mold compression techniques therefore have a series of advantages over injection processes, for example:
i) Open mold compression techniques allow the use of a wider variety of plastic materials than injection processes. Many non-suitable or completely inappropriate materials can be used for an injection process.
ii) They do not require cycle temperatures as high as in injection processes and, therefore, the energy consumption is lower, the required facilities are less complex and a much wider variety of materials can be used for inserts, reinforcements or lining layers.
iii) The plastic material is not as hot, therefore the stabilization of the part requires less time, which results in higher productivity. This also means that greater control over the quality of the parts is obtained or, in other words, for the same cycle time, the stabilization is greater and therefore better parts are obtained.
iv) Furthermore, due to the use of lower temperatures and pressures, the risk of defects in the lining is reduced. With high temperatures, the thermoplastic material can, for example, pass through the pores of the lining or cause tears in the lining in which the thermoplastic material could enter.
v) Many times it is not necessary to use a complementary protection layer.
vi) The facilities or tools used in these processes can be simpler and smaller than those used in injection processes.
vii) The use of lower temperatures can be useful for reducing cycle times.
Nevertheless, these open mold compression techniques have the drawback that the metering machine causing or generating the melted or softened thermoplastic material is usually a very large and heavy machine, which cannot be easily moved for pouring the plastic material into the cavity of the mold. It is certain that perhaps it is not necessary to move the entire machine: this type of machine can incorporate, for example, thermoplastic material feed hoppers, thermoplastic material heating means and other devices that do not have to be moved and, furthermore, an arm by means of which the thermoplastic material is introduced in the mold, which arm generally incorporates heating means to prevent the material from cooling down and a thermoplastic material driving system, for example a worm screw. However, this arm would have to move and would still be a very large and heavy element. Furthermore, the mold generally incorporates guiding columns making it difficult for the arm of the metering machine to access the cavity of the mold to be filled with the thermoplastic product (the columns are usually arranged between the lower part and the upper part of the mold so as to allow the guided movement of the upper part for the pressure closing of the mold).
These open mold techniques further have the drawback that the forming pressure is the closing pressure of the mold and the temperature of the thermoplastic material is lower than the temperature used in the injection process, therefore a poor filling of the mold may occur (for example, if the material is very cold, even though it exerts pressure, the material does not completely fill the mold) and the obtained part may be less packed than when using injection techniques and, therefore, have worse mechanical characteristics. This means that to obtain parts with similar features as those obtained by injection, the parts must incorporate reinforcing members.
These reinforcing members can consist of, for example, meshes or screens which are placed inside the open mold, then introducing the plastic material in the mold on the screen or mesh.
There are a number of publications reflecting processes for obtaining products comprising reinforcing members such as meshes or screens, for example, DE-A-19922202, DE-A-3642063, US-A-2002/0035796, EP-A-1529618, JP-A-2006-289651, JP-A-3-009836, US-A-5135694, JP-A-2003-039580, JP-A-56-078953 and JP-A-2001-301074.